The present disclosure relates to a transfer unit which transfers a toner image formed on an image carrier to a sheet and to an image forming apparatus including the transfer unit.
In an image forming apparatus, such as a copying machine and a printer, a toner image formed on an image carrier, such as a photosensitive drum and an intermediate transferring belt, is transferred to a sheet from the image carrier at a transferring nip formed between the image carrier and a transfer member.
The sheet is guided to the transferring nip along a guide. The guide is configured to guide the sheet to the transferring nip with a constant conveying posture. When the sheet is guided with the constant conveying posture, the guide receives a load from the sheet. As the size of the image forming apparatus is reduced in recent years, a curvature of a conveying path of the sheet becomes large. As a result, a stiffness of the sheet has a large effect on the conveying posture. In a case of a sheet having a large rigidity such as a thick paper, the load received by the guide increases because of its strong stiffness.
There is an image forming apparatus provided with a lever which is turned to press the transfer member against the image carrier. The lever is pressed by a turnable conveying unit and then turned to press the transfer member against the image carrier.
An example in which the transfer member is pressed by the lever will be described with reference to FIG. 7. In this example, a transfer roller 101 as the transfer member and the guide 102 are supported by a holder 103. The levers 104 support both end portions of a rotating shaft 101b of the transfer roller 101 in a rotatable manner. The lever 104 has a bearing part 108 and a spring receiving part 109. The bearing part 108 and the spring receiving part 109 are located on one side and the other side across the rotating shaft 101b. The bearing part 108 is supported by a boss 107 provided on the conveying unit (not shown) in a rotatable manner. The spring receiving part 109 is biased by a coil spring (not shown) provided on the conveying unit.
When the conveying unit is turned, the levers 104 are turned around the bosses 107 and presses the rotating shaft 101b of the transfer roller 101 such that the transfer roller 101 is pressed against the intermediate transferring belt 106 to form the transferring nip 111. The holder 103 and the lever 104 respectively have turning stoppers 113 and 114 both of which are engaged with each other to prevent the holder 103 from being turned with respect to the lever 104.
In a case of the example shown in FIG. 7, when the sheet passes through the transferring nip 111, if the sheet has a strong stiffness, the load (refer to a white bulk arrow in FIG. 7) received by the guide 102 from the sheet becomes large, and then a moment which turns the holder 103 around the rotating shaft 101b in the counterclockwise direction in FIG. 7 (refer to an arrow in FIG. 7) is generated. Then, depending on a mounting backlash and a deflection of each member, the holder 103 may be turned to displace the guide 102. However, because the holder 103 and the lever 104 respectively have the tuning stoppers 113 and 114, the holder 103 is only turned until the turning stoppers 113 and 114 are engaged with each other.
However, depending on sizes and shapes of the holder 103 and the levers 104, it is necessary to form the turning stoppers 113 and 114 near an axial center of the rotating shaft 101b. Then, if the turning stoppers 113 and 114 may have dimension errors, a turning angle of the holder 103 until the tuning stoppers 113 and 114 are engaged with each other becomes large, and thus the holder 103, that is, the guide 102 is considerably displaced. In addition, because the load received by the guide 102 is transferred to the levers 104 through the holder 103, dimension error and rigidity of the holder 103 is also affected on the displacement of the guide 102.
If the guide 102 may be displaced, the conveying posture of the sheet is varied such that it becomes difficult to guide the sheet to the transferring nip 111 stably. As a result, image failure may be occurred. Alternatively, if the guide 102 and the holder 103 may be made using high rigid material in order to withstand the increased load, the size of each member may become large and a material cost may be increased.